A 30-second spot on NBC’s warhorse comedy costs nearly $456,000 – vaulting it past “Survivor” as TV’s most expensive show, according to a report in Advertising Age magazine.

And with an average of 20 30-second spots airing on each “Friends” episode, that means NBC is raking in nearly $9.3 million from 8 to 8:30 p.m. every Thursday.

That is why the six stars of “Friends” are making $1 million each per episode. In short, the network can afford it.

But why is “Friends” so expensive when “CSI” was TV’s top-rated show last season – and averaged over 30 million viewers in its season premiere last week?

The fact that “Friends” is likely to end after this season has a lot to do with it, says Ad Age, pointing to sentimentality.

And it doesn’t hurt that ratings for “Friends” shot up 21 percent last year in the 18-49 demographic coveted most by advertisers (who surmise that younger people have more disposable income and buy more stuff).

“I’m sure the demos are the key to this equation,” says Tom DeCabia of PHD USA, a media-buying company.

“The young demos that ‘Friends’ delivers vs. ‘CSI’ . . . are important to advertisers.

“And keep in mind that ‘CSI,’ which is a one-hour show, has twice as many units to sell,” says DeCabia. “It’s pure supply and demand – the supply on ‘Friends’ is a lot less than on ‘CSI.’

“And there’s one more thing – a lot of it is emotion to [ad] buyers,” DeCabia says. “They identify with a particular show like ‘Friends.’ It would be like if HBO took sponsors for ‘The Sopranos’ – everyone would want to be a part of it.”

Wayne Friedman, who co-wrote the Ad Age story, also points out another factor.

“Movie companies really need to be in what they perceive as younger-skewing shows, and ‘Friends’ has a strong 18-49 demo,” Friedman says.

“And a lot of movie companies buy [ads] at the last moment, driving up the price. ”